This invention relates to hydraulically controlled arms, and in particular to a boom having a pivotally connected mast and a stick pivotally connected to the mast, wherein the positions of the mast and stick are controlled with hydraulic cylinders.
The boom system plays a very important role in heavy mobile machines, like forestry machines. In forestry work, over 50% of the machine work time is maneuvering the boom. Thus, it is important that boom controls be operable with efficiency and smooth and logical response.
In mobile machines boom systems are normally hydraulically driven. Hydraulic systems are known for their outstanding power density and ability to generate high force.
A characteristic feature of boom hydraulics is the type of control used. Boom motions are almost always accomplished by hydraulic control valves, which typically have a smooth and logical response and are low in cost. However, there are also disadvantages related to valve control. The most important of these is poor efficiency. In hydraulics, valve control is sometimes referred to as loss control, because of the high losses associated with flowing high volumes of hydraulic fluid through valves with high pressure differentials.
The speed of movement of the boom is directly related to the speed of the boom actuator, typically a hydraulic cylinder, which is affected by the magnitude of the oil flow to or from the actuator. In valve control, flow magnitude is controlled by throttling, i.e., reducing or enlarging, the oil channels in the control valve.
The flow magnitude through a valve is largely determined by two factors. One factor is the valve opening, which is the size of the port that oil has to pass through, and the second is the pressure difference across the port. In the case of turbulent flow this can be expressed by the equation:
Q=Kxc2x7Axc2x7xe2x96xa1xcex94p
where Q is flow, K is a constant related to the valve, A is the opening area of the port and xcex94p is the pressure difference across the port.
To illustrate the power loss in the valve port we can express the following equation:
PLoss=Qxc2x7xcex94p
This equation shows that the power loss increases as the flow and pressure difference increase.
In mobile machines, boom systems include multiple joints and actuators which are powered by a common pump. Referring to FIG. 1, in a typical system 10, a stick boom 20 is pivotally mounted by joint 24 to hoist boom 18, which is pivotally connected at joint 22 to the machine frame 34. A hoist actuator 14 is pivotally connected to the frame 34 at joint 26 and to the hoist boom 18 at joint 28. Stick actuator 16 is pivotally connected at joint 30 to the hoist boom 18 and at joint 32 to the stick boom 20. Stick actuator 16 is controlled by valve V1 and hoist actuator 14 is controlled by valve V2. In general, both valves V1 and V2 are supplied with pressurized hydraulic fluid from the same pump.
Typically, the actuators have different speed and load requirements. To ensure proper functioning of the system, the pump must deliver oil to the actuator valves according to at least the highest pressure demand. For valves serving a lower pressure load, a very high pressure difference can result in these valves, leading to a high power loss.
The worst case for boom system efficiency occurs when one actuator does positive work with high load thus causing a high pressure demand from the pump, while other actuators require fast motion with low load. In this situation much of the hydraulic energy is converted to heat in the low load valve ports. Another example relates to lifting and lowering of a load in a valve controlled system. If the load is first lifted upwards, the system must take energy (power) from the pump to do that. When the load is lowered back down, the system loses that energy in the valve control port. In such a case, the lowering energy can be stored in hydraulic accumulators. However, there are disadvantages like high cost and unreliability associated with hydraulic accumulators.
Another difficulty related to boom systems is the difficulty of handling. In forest machines like feller bunchers which have joint booms (as in FIG. 1), the driver has to control multiple joints of the boom at the same time. To reach a tree, for example, the driver has to control simultaneously, at least, the hoist, the stick and the tilt actuators. The working speed and smoothness of the boom end trajectory is highly dependent on the driver""s capabilities.
One typical work cycle in forest machines is as follows:
1. Boom end (the cutting tool) is extended relatively horizontally outward to the tree to be cut.
2. The tree is cut by the tool.
3. The tree is lifted some amount.
4. The boom end holding the tree is retracted inwards.
5. The tree is felled (laid down or dumped) or the boom end is extended to the next tree to be cut if trees are to be accumulated by the head.
With the joint boom shown in FIG. 1, the boom end 12 horizontal trajectory is accomplished by maneuvering simultaneously the hoist actuator and the stick actuator. When the boom end 12 is extended, the hoist boom 18 must be driven downwards and the stick boom 20 upwards. This means that the hoist boom actuator 14 is doing negative work, that is braking work since it is falling under the influence of gravity, and the stick boom actuator 16 is doing positive work, being lifted against the effects of gravity. When the boom end 12 is retracted, the situation is the opposite: the hoist boom actuator 14 does positive work and the stick boom actuator 16 does negative work. The magnitude of the forces required depends on the load and the stroke length of the actuator. The load is mainly affected by the masses of the boom, the head (not shown) which is mounted at the end 12, and the tree(s) supported by the head.
In conventional valve controlled boom systems as illustrated schematically in FIG. 1, the braking work is accomplished by throttling the port of a control valve so as to reduce its area. Meanwhile the positive work required to be done is powered by the hydraulic pump, which has to provide pressurized oil to the other actuator via its control valve. A disadvantage of this system is that the braking energy is lost completely and is converted to heat in the valve port. In some cases, so much energy is lost that the hydraulic oil may become overheated.
The present invention improves handling of a boom system and diminishes power losses. By providing a hydraulic control system that directs the braking power of one actuator (or one set of actuators) to apply working power to a different actuator (or set of actuators), the braking power is not wasted, i.e., converted to heat, and less power is demanded from the pump to provide the working power.
In a preferred form, a hydraulic circuit for controlling the system has two valves, and the valves and actuator hydraulic circuits are set up so that one valve essentially controls horizontal movements, and the other valve can be used to control essentially vertical movements.
In a useful aspect, the gravity sides, i.e., those sides of the hoist and stick actuators which are pressurized by gravity, are connected by a common hydraulic line. Pressurizing the anti-gravity side, i.e., the side de-pressurized by gravity, of the hoist actuator extends the hoist boom and also pumps fluid from the gravity side of the hoist actuator to the gravity side of the stick actuator, which extends the stick boom. The result is that the end of the boom extends in a largely horizontal direction with the operation of only one valve, and gravity helps pump fluid to extend the stick actuator.
Retraction in a largely horizontal direction results from shifting the same valve in the opposite direction, so as to connect the anti-gravity side of the hoist actuator with tank and pressurize the anti-gravity side of the stick actuator. The gravity side of the stick actuator pumps fluid to the gravity side of the hoist actuator so that the action of both actuators contributes to retracting the end of the boom in a largely horizontal direction.
Largely vertical motion of the end of the boom is accomplished using the other valve, by extending or retracting the hoist actuator.
Thus, the invention provides a pivoting boom control system which is easier to control for largely horizontal and vertical motions and more power efficient.
These and other objects and advantages of the invention will be apparent from the detailed description and drawings.